1. Field of Invention
The invention relates to an electro-optical panel, a method for driving the electro-optical panel, an electro-optical device including the electro-optical panel, and electronic equipment including the electro-optical panel.
2. Description of Related Art
Conventional electro-optical devices, such as active-matrix liquid-crystal display panels, typically include an element substrate, a counter substrate opposed to the element substrate, and a liquid crystal interposed between the two substrates. The element substrate typically includes a plurality of data lines, a plurality of scanning lines, a matrix of pixel electrodes at locations corresponding to the intersections of the data lines and the scanning lines, and thin-film transistors (hereinafter referred to as TFTs) respectively arranged for the pixel electrodes. Typically, the counter substrate includes a counter electrode and a color filter.
FIG. 18 shows an equivalent circuit of a pixel used in a conventional electro-optical panel. As shown, the pixel includes a TFT 1, a capacitor 2 of the liquid crystal and a storage capacitor 3. Both capacitors 2 and 3 are connected to a drain electrode of the TFT 1. The TFT 1 is configured with the gate electrode thereof being connected to a scanning line 4 and with the source electrode thereof being connected to a data line 5. The liquid-crystal capacitor 2 is formed of the liquid crystal interposed between the pixel electrode and the counter electrode.
When a scanning signal (a selection voltage) is applied to the TFT 1 through the scanning line 4 in this circuit, the TFT 1 is turned on. When an image signal is applied to the pixel electrode through the data line 5 with the TFT 1 in a conductive state, a predetermined charge is stored in the liquid-crystal capacitor 2 between the pixel electrode and the common electrode. When the TFT 1 is turned off in response to a non-selective voltage applied subsequent to storage of the charge, the charge at the liquid-crystal capacitor 2 is maintained. When the amount of charge to be stored is controlled using the TFT 1, the liquid crystal varies the orientation state pixel by pixel, thereby displaying predetermined information.
Since an off resistance of the TFT 1 is finite, charge stored in the liquid-crystal capacitor 2 is gradually discharged with time. The storage capacitor 3 is arranged to increase a time constant of discharge. Charge storage characteristics of the liquid-crystal capacitor 2 are thus improved. As a result, a contrast ratio of the panel is improved, and vertical cross-talk is controlled.
Since the field frequency of the image signal is 60 Hz in a NTSC system, the electro-optical panel is typically driven at 60 Hz. Depending on applications, the electro-optical panel has a low field frequency, for example, as low as 15 Hz or 30 Hz, or a high frequency, for example, as high as 120 Hz or 240 Hz.
To write the voltage of the data line 5 to the above-referenced pixel, the TFT 1 is turned on during a selective period and the voltage of the data line 5 is written to the liquid-crystal capacitor capacitance 2 through a time constant determined by the liquid-crystal capacitor, the storage capacitor, and the on resistance of the TFT 1. With the storage capacitor 3 added to the liquid-crystal capacitor 2, write time required to write the voltage on the pixel is prolonged.
With the storage capacitor 3 added, the voltage of the data line 5 is not sufficiently written to the liquid-crystal capacitor 2 when the field frequency is high and the selective period is short. On the other hand, with the storage capacitor 3 not added, holding a write voltage is difficult when the field frequency is low with a long holding period.
In the conventional electro-optical panel, typically the storage capacitor is determined to be compatible with both the selective period and the holding period. Varying these periods in accordance with the field frequency has not been contemplated.
The invention has been developed in view of the above problem, and it is an object of the present invention to assure the writing of the voltage to the pixel and the holding of the written voltage at the pixel even if the selective period and the holding period are varied.
To achieve the above object, an electro-optical panel of one exemplary embodiment of the invention includes a first substrate having a plurality of scanning lines and a plurality of data lines formed thereon, a second substrate opposed to the first substrate, and an electro-optical material interposed between the first substrate and the second substrate. The first substrate includes a plurality of control lines respectively formed corresponding to the scanning lines. A first switching element, which is arranged at each intersection of one of the scanning lines and one of the data lines, is controlled for an on and off operation in response to a scanning signal supplied through the scanning line. The first switching element is connected between the data line and a pixel electrode. A second switching element, which is arranged at each intersection of one of the scanning lines and one of the data lines, is controlled for an on and off operation in response to a control signal supplied through the control line. The second switching element is connected between the pixel electrode and a storage capacitor.
The electro-optical panel controls the second switching element in an on and off operation in response to the control signal, thereby connecting the storage capacitor to the pixel electrode or disconnecting the storage capacitor from the pixel electrode as necessary. An electro-optical material is switched based on whether or not the storage capacitor is connected to the electro-optical material capacitance format of the pixel electrode. A time constant involved in feeding a voltage from the data line to the pixel electrode through the first switching element is determined by an on resistance of the first switching element and the pixel capacitance value. A time constant involved in holding the voltage at the pixel is determined by the off resistance of the first switching element and the pixel capacitance value. The pixel capacitance value is modified depending on whether the storage capacitor is connected to the electro-optical material capacitance. By generating the control signal in accordance with the write period and the holding period, the voltage of the data line is reliably written on the pixel, and the voltage written on the pixel is reliably held. In this way, even if the field frequency is dynamically changed, high image quality is maintained.
Preferably, in another exemplary embodiment of the invention, an electro-optical panel further includes a first converter which converts input image data into point-at-a-time image data, a second converter which converts the point-at-a-time image data into line-at-a-time image data, a data line signal supplier which supplies each data line with a data line signal that is generated in accordance with the line-at-a-time image data, and a scanning line driver which generates a scanning signal for successively selecting the scanning lines and supplies the scanning lines with the scanning signal. Since the electro-optical panel includes the drivers for driving the data lines and the scanning lines, no driver circuits need to be arranged external to the electro-optical panel. A compact design is thus implemented in an apparatus that incorporates the electro-optical panel.
An electro-optical device according to another exemplary embodiment of the invention includes the above-referenced electro-optical panel and a control signal generator which generates the control signal in accordance with a field frequency. In this electro-optical device, the second switching element is controlled to disconnect the storage capacitor when the field frequency is high and the write period is short. When the field frequency is high and the holding period is long, the second switching element is controlled to connect the storage capacitor. In this way, even if the field frequency is dynamically changed, high image quality is maintained.
The control signal generator may generate the control signal respectively corresponding to each control line, and may supply the control line with the respective control signal. In this arrangement, whether the storage capacitor is connected is determined on a control line by control line basis, and more detailed control is performed. An electro-optical device of the invention may include the above-referenced electro-optical panel, and a control signal generator which generates the control signal depending on whether an image to be displayed is a moving picture or a still picture. In this electro-optical device, the second switching element is controlled so that the storage capacitor is disconnected when the image to be displayed is the moving picture. The second switching element is controlled so that the storage capacitor is connected when the image to be displayed is the still picture. In this way, even if the field frequency is dynamically changed, high image quality is maintained.
The invention according to another exemplary embodiment, relates to an electro-optical panel and includes a first substrate having a plurality of scanning lines and a plurality of data lines formed thereon, a second substrate opposed to the first substrate, and an electro-optical material interposed between the first substrate and the second substrate. The first substrate includes a plurality of control lines respectively formed corresponding to the scanning lines. The first substrate includes a first switching element arranged at each intersection of one of the scanning lines and one of the data lines. The first switching element is controlled for an on and off operation in response to a scanning signal supplied through the scanning line, and is connected between the data line and a pixel electrode. The first substrate also includes a second switching element arranged at each intersection of one of the scanning lines and one of the data lines. The second switching element is controlled for an on and off operation in response to a control signal supplied through the control line, and is connected between the pixel electrode and a storage capacitor. The first substrate includes a plurality of control circuits which generate the control signal, to be supplied to each control line, in accordance with the scanning signal and a common control signal indicating whether an image to be displayed is a moving picture or a still picture. Since the control signal is produced on a control line by control line basis (or on a scanning line by scanning line basis) in accordance with this invention, whether to connect the storage capacitor is controlled on a control line by control line basis.
Preferably, in one exemplary embodiment of the invention, during an active period of the scanning signal, the control circuit generates the control signal for turning off the second switching element when the common control signal indicates a moving picture, and generates the control signal for turning on the second switching element when the common control signal indicates a still picture. During a non-active period of the scanning signal, the control circuit generates the control signal for holding the state of the second switching element during the active period immediately prior to this non-active period of the scanning signal.
In one exemplary embodiment of the invention, the electro-optical panel preferably includes a scanning line driver which generates the scanning signal for successively selecting the scanning lines and supplies the scanning lines with the scanning signal for only a period during which an enable signal supplied from outside remains active.
The first switching element and the second switching element are preferably thin-film transistors. The thin-film transistors are advantageously formed on a glass substrate, or other substrates.
The invention according to another exemplary embodiment relates, to an electro-optical device which switches between a moving picture and a still picture on a field by field basis, and includes the above-referenced electro-optical device. The electro-optical device also includes a common control signal generator which generates the common control signal which is a binary signal which transitions in level on a field by field basis for indicating a moving picture thereof with one signal level and a still picture with the other signal level thereof, depending on whether an image to be displayed is the moving picture or the still picture. Further included in the electro-optical device is an enable signal generator which generates the enable signal that alternates between an active period and a non-active period with a constant period wherein the enable signal is active during a moving picture display period, while, during a still picture display period, the enable signal is active in a first field and becomes non-active in one field or a plurality of fields subsequent to the first field.
In accordance with one exemplary embodiment of the invention, voltage writing and voltage holding are performed on a field by field basis during the moving picture display period. During the sill picture display period, the electro-optical panel is driven with a period equal to a combination of the write period of one field and the holding period of a plurality of fields. During the still picture display period, the ratio of voltage writing is reduced, leading to a reduction in power consumption.
Preferably, in one exemplary embodiment of the invention, the common control signal generator transitions the signal level of the common control signal within a vertical blanking period, and the enable signal generator switches between the active period and the non-active period in the enable signal within the vertical blanking period. The image quality is thus improved, because no scanning lines are selected during the vertical blanking period.
The invention, according to another exemplary embodiment, relates to an electro-optical device which switches between a moving picture and a still picture on a scanning line by scanning line basis. The electro-optical device according to this embodiment includes the above-referenced electro-optical device. The electro-optical device according to this embodiment also includes a common control signal generator which generates the common control signal which is a binary signal which transitions in level for indicating a moving picture with one signal level thereof and a still picture with the other signal level thereof, depending on whether a moving picture display area is scanned during a moving picture display period or a still picture display area is scanned during a still picture display period. The electro-optical device according to this embodiment further includes an enable signal generator which generates the enable signal that alternates between an active period and a non-active period with a constant period wherein the enable signal is active during the moving picture display period in each field, while, during the still picture display period, the enable signal is active in a first field and becomes non-active in one field or a plurality of fields subsequent to the first field.
In one exemplary embodiment of the invention, when an area of the display screen presents a moving picture while the remaining area of the display screen presents a still picture, the storage capacitor is disconnected in the moving picture display area while the storage capacitor is connected in the still picture display area. Since the enable signal remains active in the moving picture display area, the voltage of the data line is written on the capacitance of the electro-optical material by successively selecting the scanning lines. In the still picture display area, the enable signal remains active with a constant period, and voltage writing is performed during the first field. Voltage holding is then performed during fields subsequent to the first field. During the still picture display period, the ratio of voltage writing is reduced, leading to a reduction in power consumption.
Preferably, according to one exemplary embodiment of the invention, the common control signal generator transitions the signal level of the common control signal within one of a vertical blanking period and a horizontal blanking period, and the enable signal generator switches between the active period and the non-active period in the enable signal during one of the vertical blanking period and during the horizontal blanking period.
The image quality is improved, because no scanning lines are selected during the vertical blanking period and during the horizontal blanking period.
The invention, according to one exemplary embodiment relates, to a driving method for displaying an image on an electro-optical panel which includes a plurality of scanning lines, a plurality of data lines, and a matrix of first capacitors and second capacitors. Each of the first and second capacitors are arranged at an intersection of one of the scanning lines and one of the data lines. The driving method includes the steps of determining whether a field frequency of an image to be displayed is higher or lower than a predetermined frequency, writing the voltage of the data line to the first capacitor by successively selecting the scanning lines with the first capacitor disconnected from the second capacitor, when the field frequency is higher than the predetermined frequency, and writing the voltage of the data line to the first capacitor and the second capacitor by successively selecting the scanning lines with the first capacitor connected to the second capacitor, when the field frequency is lower than the predetermined frequency. In accordance with the invention according to one exemplary embodiment, whether to connect the first capacitor to the second capacitor is determined depending on the field frequency. In this way, even if the field frequency is dynamically changed, high image quality is maintained.
The invention, according to one exemplary embodiment, relates to a driving method for displaying an image on an electro-optical panel which includes a plurality of scanning lines, a plurality of data lines, and a matrix of first capacitors and second capacitors. Each of the first and second capacitors are arranged at an intersection of one of the scanning lines and one of the data lines. The driving method includes the steps of determining whether an image to be displayed is a moving picture or a still picture, writing the voltage of the data line to the first capacitor by successively selecting the scanning lines with the first capacitor disconnected from the second capacitor, when the image to be displayed is the moving picture, and writing the voltage of the data line to the first capacitor and the second capacitor by successively selecting the scanning lines with the first capacitor connected to the second capacitor, when the image to be displayed is the still picture.
In accordance with the invention, according to one exemplary embodiment, whether to connect the first capacitor to the second capacitor is determined depending on whether the image to be displayed is a moving picture or a still picture. High image quality is thus maintained even when the moving picture and the still picture are switched one from the other.
The invention, according to one exemplary embodiment, relates to a driving method for displaying an image on an electro-optical panel with a moving picture and a still picture alternating with each other on a field by field basis. The electro-optical panel includes a plurality of scanning lines, a plurality of data lines, and a matrix of first capacitors and second capacitors. Each of the first and second capacitors are arranged at an intersection of one of the scanning lines and one of the data lines. The driving method includes the steps of writing the voltage of the data line to the first capacitor by successively selecting the scanning lines with the first capacitor disconnected from the second capacitor in each field in which the moving picture is displayed, writing the voltage of the data line to the first capacitor and the second capacitor by successively selecting the scanning lines with the first capacitor connected to the second capacitor in a first field of fields in which the still picture is displayed, holding the voltage written on the first capacitor and the second capacitor with the scanning lines not selected in one field or a plurality of fields subsequent to the first field, and repeating the writing steps and the holding step with a constant period.
In accordance with one exemplary embodiment of the invention, whether to connect the first capacitor to the second capacitor is determined depending on whether the image to be displayed is a moving picture or a still picture. High image quality is thus maintained even when the moving picture and the still picture are switched one from the other on a field by field basis.
The invention, according to one exemplary embodiment of the invention, relates to a driving method for displaying an image on an electro-optical panel with a moving picture and a still picture alternating with each other on a per scanning line basis. The electro-optical panel includes a plurality of scanning lines, a plurality of data lines, and a matrix of first capacitors and second capacitors. Each of the first and second capacitors are arranged at an intersection of one of the scanning lines and one of the data lines. The driving method includes the steps of writing the voltage of the data line to the first capacitor by successively selecting the scanning lines with the first capacitor disconnected from the second capacitor in each scanning line in which a moving picture is displayed, writing the voltage of the data line to the first capacitor and the second capacitor by successively selecting the scanning lines with the first capacitor connected to the second capacitor in a first field in each scanning line in which a still picture is displayed, holding the voltage written on the first capacitor and the second capacitor with the scanning lines not selected in one field or a plurality of fields subsequent to the first field, and repeating the writing steps and the holding step with a constant period.
In one exemplary embodiment of the invention, when an area of the display screen presents a moving picture while the remaining area of the display screen presents a still picture, the second capacitor is disconnected in the moving picture display area while the second capacitor is connected in the still picture display area. Since the enable signal always remains active in the moving picture display area, the voltage of the data line is written on the first capacitor by successively selecting the scanning lines. In the still picture display area, the enable signal remains active with a constant period, and voltage writing is performed during the first field, and voltage holding is then performed during fields subsequent to the first field. During the still picture display period, the ratio of voltage writing is reduced, leading to a reduction in power consumption.
Electronic equipment of the present invention includes the above-referenced electro-optical device, and may be a view finder of a video camera, a mobile telephone, a notebook computer, a video projector, etc.